Magnetic tunnel junction (MTJ) structures with perpendicular magnetic anisotropy (PMA) are promising candidates for ultra-low energy memory and logic devices such as spin-transfer torque magnetic random access memories (STT-MRAM). Among different types of perpendicular magnetic tunnel junctions (pMTJs) (also referred to as perpendicularly magnetized magnetic tunnel junctions), heavy metal (HM)/CoFeB/MgO based structures have attracted a great deal of attention due to the advantage of having smaller STT-switching current and less material processing. In addition, PMA in HM/CoFeB/MgO can be controlled by voltage, which could lead to ultra-low energy switching (<0.1 fJ) in these structures. For practical applications, especially for MRAM smaller than 10 nm, a large tunneling magneto-resistance (TMR) in the range of several hundred percent and a strong PMA energy greater than 4 erg/cm2 are required for pMTJs. A high thermal budget over 400° C. for more than 1 hour is also required for compatibility with back end of line (BEOL) processes of CMOS technology.
Tantalum (Ta) is the most commonly used heavy metal in HM/CoFeB/MgO pMTJs due to its amorphous nature and high affinity with Boron. However, research shows that both TMR and PMA deteriorate upon annealing at temperatures above 400° C. for Ta/CoFeB/MgO junctions. Several other under layers such as Pt (Pd), Hf, Mo and W have been studied to improve thermal stability, TMR, PMA, electric field and spin orbit torque effects in pMTJs. While other materials may have superiority in one area, none offer improvement of both TMR and PMA after annealing. Furthermore, the doping of Ta buffer with N, or using a thin sacrificial Mg layer were also reported to improve PMA and TMR in pMTJs. However, improvements of pMTJs in terms of TMR, PMA and thermal stability are in demand.